1. Technical Field
The present invention relates to the field of metrology scatterometry measurements, and, more particularly, to illumination patterns in scatterometry.
2. Discussion of Related Art
Metrology targets are designed to enable the measurement of parameters that indicate the quality of wafer production steps and quantify the correspondence between design and implementation of structures on the wafer. Scatterometry overlay (SCOL) targets are periodic structures (e.g., gratings) which are used to produce diffraction patterns for metrology measurements. Diffraction patterns are produced by illuminating the periodic structures along their measurement direction (e.g., perpendicularly to the elements of the grating), as illustrated for example in FIG. 1 below.
FIG. 1 is a high level schematic illustration of prior art scatterometry illumination. Input pupil image 71 illustrates the illumination sources 90 (log intensity) at the periphery of the pupil plane, as they are arranged along the X and Y axes to enable diffraction measurements at directions X and Y of respective target 60 (illustrated is target 60 having a measurement direction along the X axis). Positioning illumination sources 90 along respective measurement axes and at the periphery of the pupil plane maximizes the incidence angle of the illumination upon target 60 in the plane defined by the measurement direction and the normal to the target's surface. Resulting image 81 depicted at the detector's (pupil) plane illustrates the log intensity of the resulting spots—spots 92Y of non-diffracted Y axis illumination sources 90, spots 92X which are zero order diffraction images of sources 90 along the X axis and spots 91X which are ±1st order diffraction images of sources 90 along the X axis (the exact identity of the two central spots of +1 or −1 order images of left or right X axis source 90 respectively depends on measurement configurations).
The disadvantage of the prior art quadruple illumination method is that the maximal diffraction angle is limited by the presence of the zero order diffracted light spots 92X. Conceptually, different masks could have been used to measure independently the diffraction of the different orders. Measuring both the first and minus first diffraction order simultaneously decreases total measurement time and machine complexity. However, when illuminations from both directions are present, the diffraction orders resulting from one aperture in the quadruple cannot overlap any of the orders of a second aperture. In the prior art quadruple arrangement, this is realized by limiting the diffraction angle (i.e., by using shorter wavelengths or longer pitch gratings), thus limiting the method's usefulness.